Experimental study of single triple-phase-boundary and platinum–yttria stabilized zirconia composite as cathodes for micro-solid oxide fuel cells

Micro-solid oxide fuel cell (μSOFC) structures with annular Pt electrodes have been fabricated by means of silicon micromachining. The annular cathodes contained a defined triple phase boundary (TPB) length, which allowed the derivation of the ionic current per length of TPB as 1.24 mA m−1, measured at 450 °C at maximal power. Considering literature values for oxygen diffusion activation on Pt, the active zone supplying atomic oxygen to the triple phase boundary was calculated to be 22 nm wide at most. The anode function was provided by a CGO layer, known to be electrically conductive at reducing conditions, with annular Pt current collector. The TPB length was increased by adding a Pt–YSZ composite cathode layer covering the complete YSZ/CGO membrane. The peak power density was increased by a factor of 5 to reach 5 mW cm−2 with an open circuit voltage of 0.68 V at 450 °C. It was observed that the Pt grains re-crystallized to large grains, leading to a loss of electrical connectivity in the composite layer. The composite cathode layer was thus inadequate to contact the complete membrane area, leading to a too large area specific resistance (ASR) in the interior of the cell.

► The width of TPB for the Pt(O2)/YSZ system is at most 22 nm wide at 450 °C.
► The current per TPB length amounts to 1.2 mA m−1 at the maximal power.
► This small value enables high power densities with nanostructured cathode layers.
► Annealing treatment at 500 °C enables pore formation, and hillock growth of Pt in Pt–YSZ composite films.